A display is a device that processes data received from a computer processor and outputs a visual display of the data onto a display screen. In a conventional computer system, a video signal is transmitted from the computer processor to the display using a video link, such as a cable. Other data, such as audio signals for example, are transmitted from the computer processor across a separate audio link to the display or to an audio peripheral device such as a speaker.
A computer processor outputs video data across the video link in a video data format. The processor also outputs audio data across the audio link in an audio data format. Thus, conventional computer systems require multiple cables to enable the computer processor to transmit video and audio signals to the display and audio devices. Similarly, additional wires or cables are required to send additional types of data from the computer processor to the display device using conventional approaches. For example, these additional types of data may be brightness control, contrast control, or control of a microprocessor within the display device, and may each require a separate cable.
Another way to send additional data over a video link is to send the data during the video blanking period using a control signal, such as the Hsync signal, to identify the blanking period. However, one lead is needed to transmit the video and additional data to the receiver, and a separate lead is needed to transmit the sync signal from the transmitter to the receiver.
This sending of additional data over a video link using a control signal assumes a constant waveform, e.g., the number of pulses within each period and the polarity of the waveform during each period are the same. However, this seemingly periodic and predictable control signal can lose its periodicity and predictability for various reasons, such as transmission errors or design characteristics. For example, if the additional data is sent assuming that the sync signal will rise at time T but then the sync signal rises at time T−1, the receiver will mistakenly expect that up to the time T the received data is additional data and not video data. Also, in some video scrambling methods, the sync signals may be scrambled such that the behavior of these signals becomes unpredictable. Such unpredictable sync signals can degrade the security of a video transmission or they can degrade the signal integrity of the video transmission itself if the sync signals are used to identify the blanking period.
Furthermore, using a sync signal to identify the blanking period fails when the video link is erratic—in other words, a non-regular or non-standard video mode. In an erratic mode, the blanking period does not occur at regular, periodic intervals. For example, a system that encodes only one pulse of Vsync cannot support a double layer supertwist nematic (DSTN) display because a DSTN display could require two Vsync pulses in a single Vsync blanking period.
Additional problems exist in conventional approaches of sending additional data from a computer to a peripheral such as a display device. For example, if the additional data is control data, such as brightness control for example, this type of data should be relatively error-free so that it can be correctly processed by the receiver. However, in the context of video signals, a conventional control signal transmission channel does not provide the bandwidth to transmit the error status of the additional data as fast as the video channel. Therefore, if the video channel transmits video signals at rates of 25–165 Mega-Hertz (MHz), but the control signal channel transmits the additional signals and error status signals at a maximum rate of 0.4 MHz, the receiver will not be able to detect errors in a timely fashion. Also, the transmission of additional data can be limited by the performance of the receiver, which would have to monitor the additional channel link to detect the error codes.